Umibōzu, meaning 'Sea Bonze', is a sea spirit in Japanese mythology. The name 'sea bonze' actually means 'Buddhist monk from the sea'. This leads many people to believe that they are actually the souls of drowned Buddhist monks since the exhibit the shaven head and typically appear to be praying.
The Umibōzu is said to live in the ocean and capsize the ship of anyone who dares speak to it. Alternatively, it's a yokai that appears to fishermen and castaways. They have skin that varies from grey to black, bald heads, a torso that appears to be made out of storm clouds, and serpentine limbs.
It is said that if angered, they ask that the crews provide a barrel that it proceeds to fill with sea water to drown them. To avoid this disastrous fate, it is necessary to give him a bottomless barrel.